Widgets: create maven build

Remove dependency on javax.mail.internet

New projects:
* org.eclipsescout.demo.widgets.parent
* org.eclipsescout.demo.widgets.ui.rap.product
* org.eclipsescout.demo.widgets.ui.swing.product
* org.eclipsescout.demo.widgets.ui.swt.product

add maven nature to all projects
Merge branch 'master' into tmp_master